know someone or something inside out

know (someone or something) inside out

To know everything or nearly about someone or something; to be thoroughly familiar with someone or something. I've read this book so many times that I know it inside out. If you study this material for weeks, you'll know it inside out by the test date. I just think you should know a person inside out before you decide to marry them.
See also: inside, know, out

know someone or something inside out

know someone or something very thoroughly.